It's also the transport hub for Belgium, so if you want to travel to Brugge, Gent, Antwerpen, or wherever, you will probably have to pass through it anyway.
If you land in Brussels, you just have to get the underground from the airport to the main train station, then a train to your final destination.
If you fly into Charleroi, you have to get a train to Brussels and then another to wherever you want to go.